Planet Nine is a proposed large planet in the outer Solar System. The term was first used widely in 2016 when astronomers Konstantin Batygin and Mike Brown suggested that clustering in the orbits of distant trans-Neptunian objects (TNOs) pointed to the gravitational pull of an unseen massive planet.
If Planet Nine exists, models suggest it may have a mass between five and ten times that of Earth, orbit at a distance of 300 to 800 astronomical units (AU), and follow a highly elongated and inclined orbit. Its orbital period could be thousands of years.
# Evidence Evidence comes from the unusual clustering of certain TNOs whose orbits appear shepherded by an unseen massive body. Simulations show that a planet in specific resonant orbits could maintain such alignments over billions of years.
The best candidate yet for the elusive Planet Nine has been spotted in two deep infrared surveys taken 23 years apart. If this mystery object really is Planet Nine, it would have a mass greater than Neptune, and currently be about 700 times farther from the sun than Earth is - arxiv.org 
Direct evidence, however, has not been found. Large sky surveys and targeted searches have yet to identify a definitive candidate. Some infrared detections have been put forward, but these do not yet fit the predicted patterns and remain unconfirmed.
Recent approaches emphasise searching for Planet Nine’s faint heat signature rather than reflected light. Infrared surveys using data from telescopes like AKARI have identified candidates with the expected thermal profile, though follow-up is needed. The James Webb Telescope has not yet been linked directly to a Planet Nine discovery, though its instruments may aid in related outer Solar System studies.
The forthcoming Vera Rubin Observatory, with its deep, wide-field survey, is expected to be especially powerful. If Planet Nine exists within the predicted range, Rubin could reveal it in its first years of operation. If no detection is made, the hypothesis may weaken significantly.
- universetoday.com
- livescience.com
# References
- Planet Nine on bbcearth.com
- Evidence for Planet nine - caltech.edu
- Planet-X - science.nasa.gov
- The Planet Nine Hypothesis (2019) - arxiv.org
- Astronomers identify first 'good' candidate for controversial Planet Nine (2025) - livescience.com
for a list of reference link.